1. Field of the Invention
This invention generally relates to radio frequency (RF) power amplifiers that operate from a non-constant supply voltage.
2. Description of the Related Art
Wireless handsets are an example of a portable device that often includes one or more such RF power amplifiers. The RF power amplifier (PA) performs the function of amplifying the RF signal from several mili-Watts to powers between 0.5 to 3 Watts. This conditions the signal to a level suitable for transmission by the antenna. The high transmission powers involved require high power to be supplied from the battery. It is desirable to minimize the current that the RF PA draws from the battery so as to prolong battery life. Many methods are used to improve battery current over the range of transmitted powers required during normal operation of a wireless handset. One common technique is to employ a DC/DC converter to reduce the supply voltage provided to the RF PA to a level consistent with the current transmit power level. Wideband Code Division Multiple Access (WCDMA) systems have a specification known as DG09, which breaks the transmitted powers into a probability distribution function. A figure of merit can then be calculated for the battery current during normal operation of a handset. When the handset is transmitting at full power the converter might step up the voltage available from the battery to a larger value or simply pass this value on to the RF PA at a near 1:1 ratio. At lower powers the DC/DC converter converts the available battery voltage to a reduced voltage, where the PA operates with improved efficiency, so that the average current being drawn from the supply is reduced. This latter condition is particularly important since wireless handsets spend a significant portion of their operating time at lower powers levels.
The above is an example where the supply to the PA is purposely varied in order to improve the overall handset system's battery life. In another class of examples undesirable variations in the supply to a PA might be present. For example, battery voltages vary under nominal use between start of life, nominal, and end-of-life and their charging value is often significantly higher than the average. The performance of RF PAs in metrics such as output power, linearity, current, and gain typically varies widely as the supply voltage varies.